I thought that larger files were more likely to cause fragmentation, as
there are more bytes to fit in...

but i guess your point is that the gaps from deleted files will be large, so
there will be less fragmentation because of that?

There is no point in copying a load of files on, then defraging - you will
most likely end up with it moving those files and taking ages... best to
defrag before copying files, then all news files will be written already
defragged, as there will only be one lump of space at the end of the disk..